There are methods today which are directed to detecting dirt on surfaces or quantifying the contamination degree on such surfaces. For instance in U.S. Pat. No. 5,828,460 there is disclosed a method for inspection of metallic surfaces before subsequent treatment thereof, such as painting. Several light sources are said to illuminate the inspected surface with light beams having different wave lengths, reflected radiation is detected and converted to electrical signals, the electrical signals are processed digitally, and the results of processing are displayed. Furthermore, methods like the one disclosed above are known to be used in e.g. the vehicle industry. In U.S. Pat. No. 6,320,654 there is e.g. disclosed a method of detecting selected surface defects of a vehicle body, the method comprising conveying a plurality of said vehicle bodies along a line of travel; irradiating sequential transverse sections of said vehicle bodies at selected angles with grid patterns of light; optically detecting reflective grid patterns of light from said transverse sections; processing data representing said reflective grid patterns, utilizing triangulation techniques and accounting for three-dimensional phase shifts, to generate composite detected data, comparing said composite data with selected reference data representing relevant defect surfaces of selected vehicles bodies and generating identification data representing coordinates of relevant defect surfaces of said vehicles bodies; and marking relevant defect surfaces on said vehicle bodies responsive to said coordinates of said identification data.
Likewise, in WO87/00629 there is disclosed a surface inspection apparatus for inspecting e.g. the paint surface of a motor car, the apparatus comprising laser means providing a beam of radiation, scanning means for scanning the beam across the surface, retro-reflective material being provided to reflect radiation reflected from the surface back along the incident beam path, the apparatus including the retro-reflective material being mounted as a unit to be moved over the surface of the motor car by means of a robot. Analysis of the light signal is said to indicate defects such as scratches, paint inclusions, orange peel, dry spray, dents and gloss defects.
Furthermore, in U.S. Pat. No. 5,844,801 there is disclosed a method of manufacturing of a vehicle body, the method comprising inter alia identifying a defective area of a surface by comparing a degree of surface distortion with a predetermined decision value, thereby determining whether said surface distortion is acceptable or not.
One aim of the present invention is to provide an improved method for detection of contamination on a surface, which method is very effective and still easy to use. Furthermore, the method according to the present invention is intended to effectively detect contaminants which are often found or used heavily today. Such examples are given below.